Finite cohesion due to chain entanglement in polymer melts. Issue 14 (2nd March 2016)

• Record Type: Journal Article
• Authors: Cheng, Shiwang; Lu, Yuyuan; Liu, Gengxin; Wang, Shi-Qing
• Is Part Of: Soft matter. Volume 12:Issue 14(2016)
• Page Start: 3340
• Page End: 3351
• Publication Date: 2016-03-02
• Subjects: Soft condensed matter -- Periodicals; 530.413
• DOI: 10.1039/c6sm00142d
• Languages: English
• ISSNs: 1744-683X

Abstract

Abstract : Qualitative differences between experiments and theories suggest a finite cohesion that prevents free chain retraction in entangled polymer melts. Abstract : Three different types of experiments, quiescent stress relaxation, delayed rate-switching during stress relaxation, and elastic recovery after step strain, are carried out in this work to elucidate the existence of a finite cohesion barrier against free chain retraction in entangled polymers. Our experiments show that there is little hastened stress relaxation from step-wise shear up to γ = 0.7 and step-wise extension up to the stretching ratio λ = 1.5 at any time before or after the Rouse time. In contrast, a noticeable stress drop stemming from the built-in barrier-free chain retraction is predicted using the GLaMM model. In other words, the experiment reveals a threshold magnitude of step-wise deformation below which the stress relaxation follows identical dynamics whereas the GLaMM or Doi–Edwards model indicates a monotonic acceleration of the stress relaxation dynamics as a function of the magnitude of the step-wise deformation. Furthermore, a sudden application of startup extension during different stages of stress relaxation after a step-wise extension, i.e. the delayed rate-switching experiment, shows that the geometric condensation of entanglement strands in the cross-sectional area survives beyond the reptation time τ d that is over 100 times the Rouse time τ R . Our results point to the existence of a cohesion barrier that can prevent free chain retraction upon moderate deformation in well-entangled polymer melts.